(https://www.airgradient.com/blog/2020/08/25/the-airgradient-...)
The linked DIY instructions give you a basic board with PM 2.5 particles, temp, humidity, and CO2. It is built on a Wemos D1 mini.
The founder (Achim) is very helpful.
If you wanted to use these plans to get started (including the software to power the Wemos), then buy whatever additional sensors you might like to add, it should be fairly simple.
